Add support for dynamical allocation of imx-keypad on mx5 platform.

After moving to dynamically registration of the keypad, the keypad clock
name needs to change accordingly.

The reason is that the original mx5 keypad platform_device id was 0,
now we use id=-1 as per arch/arm/plat-mxc/devices/platform-imx-keypad.c.

Tested keypad successfully on a MX51_3DS board.
Signed-off-by: NFabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: NSascha Hauer <s.hauer@pengutronix.de>